Diesel engines are known to be significantly more fuel efficient than their gasoline counterparts. Therefore, the introduction of diesel engine off-road systems (e.g., industrial or household equipment, such as heavy machinery and lawnmowers) is highly desirable.
The high fuel efficiency of diesel engines results in oxygen-rich (i.e., “lean”) exhaust that also contains NOx gases. Although NOx gases can be efficiently removed from oxygen-poor exhausts, as produced in gasoline engine emissions, the removal of NOx gases in diesel engine lean emissions continues to be a significant challenge. In particular, the standard three-way catalyst works well for gasoline engines but does not meet EPA regulatory requirements when used for diesel engines.
In order to meet EPA regulatory requirements for diesel engines, extensive efforts are under way to find catalysts that can effectively treat NOx in the oxygen-rich emissions emitted by diesel engines. The leading approach for reduction of NOx in diesel emissions is selective catalytic reduction (SCR). In SCR, ammonia or urea is often employed as a reductant. The best known NH3-SCR catalysts for NOx reduction under the lean environment of diesel engine emissions are Cu-ZSM-5 and Fe-ZSM-5. These have been shown to function effectively only within narrow temperature ranges. Cu-ZSM-5 generally exhibits a better NOx reduction activity at lower temperatures while Fe-ZSM-5 exhibits better activity at higher temperatures. Thus, a combination of Cu-ZSM-5 and Fe-ZSM-5 zeolites (i.e., as a heterogeneous mixture) has been used in an effort to effectively treat NOx within a broadened temperature range.
Although the Cu/Fe heterogeneous system provides an improvement in emissions processing for diesel-operated passenger vehicles, the Cu/Fe heterogeneous system is significantly inadequate when applied to off-road diesel engine systems. In particular, the catalysts currently employed do not efficiently reduce NOx emissions at low temperatures, such as 150-200° C., which is more critical in off-road diesel engines than in passenger vehicles. Moreover, the Cu/Fe heterogeneous system may be adequately efficient only within separate narrow temperature ranges, e.g., a high and a low temperature range. However, particularly for off-road diesel engines that can operate under a broad range of temperatures, there would be a significant benefit in a catalyst that can operate efficiently under a wide range of temperatures (e.g., from 150° C. to 650° C.).